<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    let aa = parseInt(prompt("请输入月份"))
    switch(aa){
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
        console.log("31天")
        break;
        case 4:
        case 6:
        case 9:
        case 11:
        console.log("30天")
        break;
        case 2:console.log("28天")
        break;
    }
</script>
</html>